Most silver halide color photographic elements form multicolor images in the element by subtractive color mixing. This involves the formation of yellow, magenta and cyan dye images by color development of imagewise exposed blue, green and red sensitive silver halide emulsion layers. Ideally, the subtractive dyes so formed should absorb radiation only in the region of the spectrum which is the complement of the region of exposure. Unfortunately, all dyes have some unwanted side absorptions. To correct for these unwanted side absorptions it is common practice for color negative photographic elements to employ one or more colored masking couplers. These couplers have a color which is similar to the unwanted side absorption of one of the dyes formed from one of the image couplers. The color of the masking coupler is destroyed in the areas of the image where the dye with unwanted side absorptions is formed. The way in which colored masking couplers are employed to correct for the unwanted side absorption is described in more detail in J. Phot. Soc. Am. 13, 94(1947), J. Opt. Soc. Am. 40, 166(1950) and J. Am. Chem. Soc. 72, 1533(1950).
A preferred class of colored masking couplers are the 4-phenylazo-5-pyrazolones which correct for the unwanted yellow side absorption of magenta dye-forming couplers. Such couplers have found widespread use in color photographic elements. Nevertheless, it has been reported, for example in U.S. Pat. Nos. 4,070,191, 4,163,670, and 5,219,719, that the coupling activity of this class of couplers can be increased if the 4-phenylazo group is substituted in certain ways. Among the phenylazo groups suggested in these patents are 2'-hydroxy-4'-substituted phenylazo groups. While such groups provide masking couplers with good activity and desired hues, they are difficult to synthesize because of the presence of the 4'-substituent. Although such a substituent can provide a desired hue to the colored coupler, its presence complicates the synthesis and decreases the yield of coupler. Specifically, during the formation of the precursor for the phenylazo group, not only is there formed the desired isomer in which the 4'-substituent is meta to the 2'-hydroxy group and para to the azo group precursor, but there are formed two undesired isomers in which the 4'-substituent is ortho to the azo group precursor. A yield reducing separation step is required to isolate the desired precursor. It would be desirable to provide colored pyrazolone masking couplers that not only have good coupling activity and desirable hues but which can be obtained in good yields by simple syntheses.